In accordance with high integration of various devices represented by VLSI, TFT liquid crystals and the like, cleanliness levels of substrate surfaces used for these devices are demanded to be higher and higher. Contaminations with various materials disturb to achieve higher cleanliness, and among the contaminations, a metallic contamination particularly deteriorates electric properties of a device, and it is therefore necessary to utmostly lower a metal impurity concentration on a substrate surface for forming a device in order to prevent the above-mentioned deterioration. For this purpose, it is generally conducted to clean a substrate surface with a cleaning agent.
Heretofore, for this type cleaning agent, there are generally used water, electrolyzed ionic water, acid, alkali, oxidizing agent, surfactant aqueous solution or organic solvents and the like. The cleaning agent is demanded not only to have excellent cleaning performances but also to have an impurity concentration of such an extremely low level as to prevent a substrate from being contaminated with a metal impurity from the cleaning agent. In order to satisfy these demands, cleaning chemicals for semiconductors are highly purified, and a metal impurity concentration in the chemicals immediately after purification reaches a level of hardly detectable by present analytical techniques.
Although an impurity in a cleaning agent is lowered to such a low level as to be hardly detectable, it is still difficult to provide a highly clean substrate surface. This is because it can not be avoided that a cleaning agent itself is contaminated with a metal impurity removed from a substrate in a cleaning tank. Thus, a metal impurity separated from a metal surface is incorporated into a cleaning agent and the metal impurity in the contaminated cleaning agent is then deposited on a substrate (reverse contamination).
In a step of cleaning semiconductors, cleaning (SC-1 cleaning) with "ammonia+hydrogen peroxide+water" solution is widely used (see RCA Review, p 187-206, June (1970) etc.). This cleaning is usually conducted at 40-90.degree. C., and the composition ratio of a cleaning solution usually used is (30 wt % aqueous ammonia):(31 wt % hydrogen peroxide):(water)=0.05:1:5 to 1:1:5. However, this cleaning method has high performances to efficiently remove particles and organic materials, but when metals such as Fe, Al, Zn and Ni are present even in a very small amount in the solution, they are deposited on a substrate surface, thus raising a problem of reverse contamination. For this purpose, in the step of cleaning semiconductors, after cleaning with "ammonia+hydrogen peroxide+water" solution, cleaning with an acid cleaning agent such as "hydrochloric acid+hydrogen peroxide+water" solution (SC-2 cleaning) is usually conducted to remove metal contamination on a substrate surface.
Therefore, in the cleaning step, a technique to prevent the reverse contamination has been demanded in order to stably and efficiently provide a highly clean surface.
Further, a problem of deposition of metal impurities in a liquid onto a substrate surface is generally a large problem not only in the cleaning step but also in substrate surface treatment steps using a solution such as an alkali etching step of a silicon substrate, an etching step of a silicon oxide film with dilute hydrochloric acid, and the like. In the etching step with dilute hydrofluoric acid, when noble metal impurities such as Cu and Au are present in the solution, they are deposited on the silicon surface and extremely deteriorate electric properties of devices such as carrier lifetime. Although, in the alkali etching step, when a small amount of metal impurities such as Fe and Al are present in the solution, they are easily deposited on the substrate surface and adversely affect on its quality. Thus, a technique to prevent contamination in a surface treatment step with a solution is strongly demanded.
In order to solve these problems, there is proposed a method for preventing metal impurities from being deposited on a substrate surface by adding a complexing agent such as a chelating agent to a surface treatment agent to trap the metal impurities in the solution as stable water-soluble complexes. For example, JP-A-50-158281 proposes to prevent deposition of metal impurities on a semiconductor substrate surface by adding a complexing agent such as ethylenediaminetetraacetic acid (EDTA) or ammonium cyanide to a tetraalkylammonium hydroxide aqueous solution. JP-A-3-219000 proposes a chelating agent such as catechol and Tiron, JP-A-5-275405 proposes a phosphonic acid type chelating agent or a complexing agent such as condensed phosphoric acid and JP-A-6-163495 proposes a complexing agent such as hydrazone derivative, and they are respectively added to an alkaline cleaning solution such as "ammonia+hydrogen peroxide+water" solution to prevent metal impurity deposition on a substrate, thereby providing a substrate surface not contaminated with particles, organic materials and metals.
However, when these complexing agents are added, deposition of a specific metal (such as Fe) can be prevented or a removal effect can be recognized, but there are problems that the effect of the above-mentioned complexing agent is extremely small with regard to metals other than Fe (such as Al) which easily contaminate a treating solution or a substrate and that the effect can not be sufficiently achieved even by adding a large amount of complexing agents. In order to solvent these problems, JP-A-6-216098 proposes to clean a substrate with "ammonia+hydrogen peroxide+water" cleaning solution containing a chelating agent such as a phosphonic acid type chelating agent and then to rinse with a hydrofluoric acid aqueous solution of at least 1 ppm. According to this method, since the cleaning solution containing the phosphonic acid type chelating agent does not substantially reduce Al contamination on the substrate surface, Al is removed by etching with the hydrofluoric acid aqueous solution of at least 1 ppm at the later step. Thus, the effect achieved by the conventional method for preventing metal deposition is not satisfactory, and the metal contamination must be removed at the later stage when the substrate is required to be cleaner. Consequently, the number of steps must be increased, thereby making a production cost large.
Under these circumstances, various complexing agents have been tried to be added in order to prevent a substrate surface from being contaminated with metal impurities from a surface treatment composition, but a satisfactory improvement can not be made and a satisfactory technique for preventing contamination can not be achieved up to now.
The present invention has been made to solve the above-mentioned problems, and provides a surface treatment composition which prevents a substrate surface from being contaminated with metal impurities from the surface treatment composition and stably produces an extremely clean substrate surface, and also provides a method for treating the surface of a substrate by using the same.